Common-ground non-isolated photovoltaic inverter

Abstract

The invention discloses a common-ground non-isolated photovoltaic inverter, which is based on a topological structure of the common-ground non-isolated photovoltaic inverter, realizes extremely high boosting capacity of a direct current-direct current unit, realizes wide-range voltage input, reduces the requirement on illumination intensity, and eliminates common-mode drain voltage and leakage current from the structural level. The invention provides a control strategy for suppressing the loss of power grid voltage and output direct current, maintaining the stability of grid-side current, reducing the requirement of a circuit on a capacitance value, avoiding the use of an electrolytic capacitor and prolonging the service life, and provides a control strategy for suppressing the loss of power grid voltage and output direct current and maintaining the stability of grid-side current.

Description

Technical Field

[0001] The present application relates to the field of photovoltaic technology, and in particular to a common-ground non-isolated photovoltaic inverter. Background Art

[0002] Photovoltaic micro-inverter is a new type of photovoltaic grid-connected device. It connects the inverter directly to the photovoltaic module, and each module can work at the maximum power point, thereby improving the overall efficiency and power generation of the system. Compared with traditional centralized or string inverters, micro-inverters can achieve maximum power point tracking (MPPT) at the module level, and maximize the overall output power by optimizing the output power of each module. In addition, micro-inverters are safer and can completely solve the fire hazards caused by high-voltage arcing.

[0003] However, existing non-isolated photovoltaic grid-connected inverters have problems such as leakage current, power ripple and harmonic current. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a common-ground non-isolated photovoltaic inverter that integrates the traditional boost converter with the switching capacitor technology to obtain a structure in which the neutral point of the power grid is directly connected to the negative pole of the photovoltaic array, thereby eliminating leakage current. At the same time, since the present invention has extremely strong boosting capability, a wide range of photovoltaic power input can be achieved, thereby solving the problems of leakage current, power supply pulsation and harmonic current existing in non-isolated photovoltaic grid-connected inverters.

[0005] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: a common ground type non-isolated photovoltaic inverter, comprising a DC-DC boost conversion unit and a DC-AC inverter step-down unit connected in sequence;

[0006] The DC-DC boost conversion unit includes a photovoltaic DC input power supply V in , a DC boost unit and an intermediate DC capacitor C1, wherein the DC boost unit includes a diode D1, a diode D2, an energy storage capacitor C2, a boost inductor L1, a freewheeling inductor L2, and switch tubes S2, S3, and S5;

[0007] The DC-AC inverter step-down unit includes an intermediate DC capacitor C1, a switch tube S1, a switch tube S2, a switch tube S4, a switch tube S5, a filter inductor L0 and a voltage stabilizing capacitor C0;

[0008] In the DC-DC boost conversion unit, the photovoltaic DC input power source V inThe positive electrode of the boost inductor L1 is respectively connected to the positive end of the energy storage capacitor C2 and one end of the boost inductor L1, the other end of the boost inductor L1 is respectively connected to the anodes of the diode D1 and the diode D2, the negative end of the energy storage capacitor C2 is respectively connected to the cathode of the diode D1 and one end of the freewheeling inductor L2, the other end of the freewheeling inductor L2 is respectively connected to the cathode of the diode D2, the drain of the switch tube S3 and the source of the switch tube S5, the drain of the S5 switch tube is connected to the positive end of the DC capacitor C1, and the source of the switch tube S2 is connected to the negative end of the DC capacitor C1;

[0009] In the DC-AC inverter step-down unit, the positive end of the intermediate capacitor C1 is connected to the drain of the switch tube S4 and the drain of the switch tube S5 respectively, the secondary end of the intermediate capacitor C1 is connected to the drain of the switch tube S1 and the drain of the switch tube S2, the source of the switch tube S4 and the drain of the switch tube S1 are both connected to the positive end of the filter inductor L0, the negative end of the filter inductor L0 is connected to the positive end of the voltage stabilizing capacitor C0, and the negative end of the voltage stabilizing capacitor C0 is connected to the drain of the switch tube S2 and the source of the switch tube S3.

[0010] Furthermore: the common ground type non-isolated photovoltaic inverter includes 6 working modes:

[0011] Working mode 1: switch tubes S2, S3 and S4 are turned on, and switch tubes S1 and S5 are turned off;

[0012] In working mode 1, the photovoltaic DC output power v in Under the action of , the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, the diode D1 and the diode D2 are turned off by the reverse voltage, and the intermediate capacitor C1 transfers the energy to the AC output side;

[0013] Working mode 2: switch tubes S1, S2 and S3 are turned on, and switch tubes S4 and S5 are turned off;

[0014] In working mode 2, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged.

[0015] Working mode 3: switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off due to reverse voltage.

[0016] In working mode 3, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit;

[0017] Working mode 4: switch tubes S1, S3 and S5 are turned on, switch tubes S2 and S4 are turned off, and D1 is turned off due to reverse voltage.

[0018] In working mode 4, the photovoltaic DC power supply charges the boost inductor L1 and the freewheeling inductor L2, and the current gradually increases; the intermediate capacitor C1 transfers energy to the AC output side, and the current of the filter inductor L0 continues to increase;

[0019] Working mode 5: the switch tubes S1, S2 and S3 are turned on, and the switch tubes S4 and S5 are turned off;

[0020] In working mode 5, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged.

[0021] Working mode 6: switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off due to reverse voltage.

[0022] In working mode 6, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit.

[0023] Further: the common-ground type non-isolated photovoltaic inverter also includes a control system;

[0024] The control module includes an independent first submodule and a second submodule, wherein the first submodule and the second submodule respectively control a DC-DC boost duty cycle d1 and a DC-AC inverter buck duty cycle d2;

[0025] The DC-DC boost duty cycle d1 is used to control the DC-DC boost conversion unit, increase the voltage of the intermediate capacitor C1 to a set target value, and eliminate the even-order current ripple on the DC side;

[0026] The DC-AC inverter step-down duty cycle d2 is used to control the DC-AC inverter step-down unit to invert and step down the voltage of the intermediate capacitor C1 to AC power with an effective value of 220V and 50Hz, and eliminate odd harmonics of the output AC power.

[0027] Furthermore: the method for obtaining the DC-DC boost duty cycle d1 comprises the following steps:

[0028] S1, sampled output AC voltage v o , and then pass through the band-stop filter to remove the secondary ripple and the reference value of the output voltage v oref *Compare and get the difference;

[0029] S2, send the comparison difference to the proportional integrator PI for adjustment to obtain the DC average power control duty cycle d f ;

[0030] S3, the sampled output voltage v o After the second and fourth bandpass filters respectively, and multiplied by the proportional coefficients K1 and K2 respectively, the corresponding duty ratios d1 of the second and fourth harmonics of the DC-DC boost duty ratio d1 are obtained. h1 d h2 ;

[0031] S4, the duty cycle d f d h1 and d h2 Add them together to get the required DC-DC boost duty cycle d1 after filtering out the second and fourth harmonics.

[0032] Further: the method for obtaining the DC-AC inverter step-down duty cycle d2 comprises the following steps:

[0033] A1, sampled output AC voltage v o As the α-axis, and the AC voltage v o The phase shift of one quarter cycle is used as the β axis;

[0034] A2, after αβ0 / dq transformation, we get v o The d-axis component v od and the q-axis component v oq ;

[0035] A3, take 1 as v od The reference value of v is 0. oq The reference value of v od Compared with 1, v oq After comparing with 0, the difference is sent to the proportional integral controller PI to obtain the control signal components u on the d-axis and q-axis szd and u szq , and obtain the modulation wave u of fundamental power control through αβ0 / dq inverse transformation szα *;

[0036] A4, multiplying the sampled AC output voltage vo by the sin and cos functions of odd frequencies respectively as dq decoupling;

[0037] A5, pass the dq decoupling result through the low-pass filter LPF, make a difference with 0, and send the difference to the proportional integral controller PI respectively;

[0038] A6. Multiply the output of the integral controller PI by the cosine and sin functions respectively as the dq inverse transformation, and add up the outputs to obtain the voltage V of the odd harmonics. h3 ;

[0039] A7, u szα * Add odd harmonics of V h3 Get the port voltage reference value u r , will u r Take the absolute value to get the DC-AC inverter step-down duty cycle d2.

[0040] The beneficial effects of the present invention are:

[0041] 1. Through the two sub-modules of the control unit, the six MOSFET switch tubes are controlled, so that the photovoltaic inverter switches between six working modes, thereby suppressing the ripple of the DC input current and the AC output voltage, and further improving the quality of the AC power sent to the grid through power decoupling;

[0042] 2. Based on the topological structure, a wide output voltage range is achieved and the boost capability is extremely strong, such as Figure 4 As shown in the figure, the input voltage range can be as low as 15V, which can be boosted to the intermediate capacitor voltage of 500V, and then inverter-down to 220V AC input to the grid. It also eliminates leakage voltage and leakage current at the structural level, reduces the requirements for capacitance value, does not require electrolytic capacitors, increases circuit life, and improves the power quality of the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the topology diagram of a common-ground non-isolated photovoltaic inverter.

[0044] Figure 2 This is a diagram of the six operating modes of the photovoltaic inverter proposed in the present invention.

[0045] Figure 3 This is the control block diagram of the present invention.

[0046] Figure 4 The waveform diagrams of input and output voltage and current of the inverter of the present invention with and without power decoupling function under different DC input conditions are shown. DETAILED DESCRIPTION

[0047] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0048] like Figure 1As shown, in one embodiment of the present invention, a common ground type non-isolated photovoltaic inverter is provided, comprising a DC-DC boost conversion unit and a DC-AC inverter buck unit connected in sequence;

[0049] The DC-DC boost conversion unit includes a photovoltaic DC input power supply V in , a DC boost unit and an intermediate DC capacitor C1, wherein the DC boost unit includes a diode D1, a diode D2, an energy storage capacitor C2, a boost inductor L1, a freewheeling inductor L2, and switch tubes S2, S3, and S5;

[0050] The DC-AC inverter step-down unit includes an intermediate DC capacitor C1, a switch tube S1, a switch tube S2, a switch tube S4, a switch tube S5, a filter inductor L0 and a voltage stabilizing capacitor C0;

[0051] In the DC-DC boost conversion unit, the photovoltaic DC input power source V in The positive electrode of the boost inductor L1 is respectively connected to the positive end of the energy storage capacitor C2 and one end of the boost inductor L1, the other end of the boost inductor L1 is respectively connected to the anodes of the diode D1 and the diode D2, the negative end of the energy storage capacitor C2 is respectively connected to the cathode of the diode D1 and one end of the freewheeling inductor L2, the other end of the freewheeling inductor L2 is respectively connected to the cathode of the diode D2, the drain of the switch tube S3 and the source of the switch tube S5, the drain of the S5 switch tube is connected to the positive end of the DC capacitor C1, and the source of the switch tube S2 is connected to the negative end of the DC capacitor C1;

[0052] In the DC-AC inverter step-down unit, the positive end of the intermediate capacitor C1 is connected to the drain of the switch tube S4 and the drain of the switch tube S5 respectively, the secondary end of the intermediate capacitor C1 is connected to the drain of the switch tube S1 and the drain of the switch tube S2, the source of the switch tube S4 and the drain of the switch tube S1 are both connected to the positive end of the filter inductor L0, the negative end of the filter inductor L0 is connected to the positive end of the voltage stabilizing capacitor C0, and the negative end of the voltage stabilizing capacitor C0 is connected to the drain of the switch tube S2 and the source of the switch tube S3.

[0053] like Figure 2 As shown, the common ground type non-isolated photovoltaic inverter includes 6 working modes:

[0054] like Figure 2 As shown in (a), working mode 1: the switch tubes S2, S3 and S4 are turned on, and the switch tubes S1 and S5 are turned off;

[0055] In working mode 1, the photovoltaic DC output power v inUnder the action of , the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, the diode D1 and the diode D2 are turned off by the reverse voltage, and the intermediate capacitor C1 transfers the energy to the AC output side;

[0056] like Figure 2 As shown in (b), working mode 2: the switch tubes S1, S2 and S3 are turned on, and the switch tubes S4 and S5 are turned off;

[0057] In working mode 2, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged.

[0058] like Figure 2 (c) shows working mode 3: switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off by reverse voltage. Through working mode 2, the intermediate capacitor voltage and the output voltage can be independently controlled.

[0059] In working mode 3, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit;

[0060] like Figure 2 As shown in (d), working mode 4: switch tubes S1, S3 and S5 are turned on, switch tubes S2 and S4 are turned off, and D1 is turned off due to reverse voltage;

[0061] In working mode 4, the photovoltaic DC power supply charges the boost inductor L1 and the freewheeling inductor L2, and the current gradually increases; the intermediate capacitor C1 transfers energy to the AC output side, and the current of the filter inductor L0 continues to increase;

[0062] like Figure 2 As shown in (e), working mode 5 is similar to working mode 2. In working mode 5, switch tubes S1, S2 and S3 are turned on, and switch tubes S4 and S5 are turned off;

[0063] In working mode 5, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged.

[0064] like Figure 2 As shown in (f), working mode 6 is similar to working mode 3. In working mode 6, switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off due to reverse voltage.

[0065] In working mode 6, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit.

[0066] like Figure 3 As shown, the common-ground type non-isolated photovoltaic inverter also includes a control system;

[0067] The control module includes an independent first submodule and a second submodule, wherein the first submodule and the second submodule respectively control a DC-DC boost duty cycle d1 and a DC-AC inverter buck duty cycle d2;

[0068] The DC-DC boost duty cycle d1 is used to control the DC-DC boost conversion unit, increase the voltage of the intermediate capacitor C1 to a set target value, and eliminate the even-order current ripple on the DC side;

[0069] The DC-AC inverter step-down duty cycle d2 is used to control the DC-AC inverter step-down unit to invert and step down the voltage of the intermediate capacitor C1 to AC power with an effective value of 220V and 50Hz, and eliminate odd harmonics of the output AC power.

[0070] The method for obtaining the DC-DC boost duty cycle d1 comprises the following steps:

[0071] S1, sampled output AC voltage v o , and then pass through the band-stop filter to remove the secondary ripple and the reference value of the output voltage v oref *Compare and get the difference;

[0072] S2, send the comparison difference to the proportional integrator PI for adjustment to obtain the DC average power control duty cycle d f ;

[0073] S3, sampled output voltage v o After the second and fourth bandpass filtering, they are v o100Hz and v o200Hz , and multiply them by the proportional coefficients K1 and K2 respectively to obtain the corresponding duty ratios d1 of the second and fourth harmonics of the DC-DC boost duty ratio d h1 ,d h2 ;

[0074] S4, the duty cycle d f ,d h1 and d h2 Add them together to get the required DC-DC boost duty cycle d1 after filtering out the second and fourth harmonics.

[0075] The expression of the DC-DC boost duty cycle d1 is:

[0076]

[0077] Among them, k p , k i are the parameters of the proportional-integral controller.

[0078] The method for obtaining the DC-AC inverter step-down duty cycle d2 comprises the following steps:

[0079] A1, sampled output AC voltage v o As the α-axis, and the AC voltage v o The phase shift of one quarter cycle is used as the β axis;

[0080] A2, after αβ0 / dq transformation, we get v o The d-axis component v od and the q-axis component v oq ;

[0081] A3, take 1 as v od The reference value of v is 0. oq The reference value of v od Compared with 1, v oq After comparing with 0, the difference is sent to the proportional integral controller PI to obtain the control signal components u on the d-axis and q-axis szd and u szq , and obtain the modulation wave u of fundamental power control through αβ0 / dq inverse transformation szα *;

[0082] A4, multiplying the sampled AC output voltage vo by the sin and cos functions of odd frequencies respectively as dq decoupling;

[0083] A5, pass the dq decoupling result through the low-pass filter LPF, make a difference with 0, and send the difference to the proportional integral controller PI respectively;

[0084] A6. Multiply the output of the integral controller PI by the cosine and sin functions respectively as the dq inverse transformation, and add up the outputs to obtain the voltage V of the odd harmonics. h3 ;

[0085] A7, u szα * Add odd harmonics of V h3 Get the port voltage reference value u r , will u r Take the absolute value to get the DC-AC inverter step-down duty cycle d2.

[0086] The expression of DC-AC inverter step-down duty cycle d2 is:

[0087]

[0088] Among them, k p , k i are the parameters of the proportional-integral controller.

[0089] The d1 controls the DC-DC unit, increases the voltage of the intermediate capacitor C1 to the required target value, and eliminates the even-order current ripple on the DC side; d2 controls the DC-AC unit, inverts and steps down the voltage of the intermediate capacitor C1 to an effective value of 220V, 50Hz AC, and eliminates the odd-order harmonics of the output AC, reducing the impact of inverter grid-connected distortion on the grid. At the same time, the power decoupling effect is achieved, the pulsating power is stored in the intermediate capacitor C1, and the photovoltaic output voltage v in Fluctuations will not affect the AC output side, further ensuring the quality of electricity sent to the grid.

[0090] Based on the topological structure, a wide output voltage range is achieved and the boost capability is extremely strong, such as Figure 4 As shown, the input voltage range can be as low as 15V, which can be boosted to the intermediate capacitor voltage of 500V, and then inverter-down to 220V AC input to the grid. The leakage voltage and leakage current are eliminated from the structural level, the requirements for the capacitance value are reduced, and no electrolytic capacitor is required, which increases the circuit life and improves the power quality of the grid.

[0091] based on Figure 3 The control block diagram can simultaneously achieve harmonic suppression of DC input current and AC output voltage, reduce circuit loss and pressure, and improve the power quality on the grid side. It also realizes the function of power decoupling, such as Figure 4 As shown, the current fluctuation is greatly reduced when power decoupling is added compared to when power decoupling is not added.

[0092] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A common ground type non-isolated photovoltaic inverter, characterized in that: It includes a DC-DC boost conversion unit and a DC-AC inverter step-down unit connected in sequence; The DC-DC boost conversion unit includes a photovoltaic DC input power supply V in , a DC boost unit and an intermediate DC capacitor C1, wherein the DC boost unit includes a diode D1, a diode D2, an energy storage capacitor C2, a boost inductor L1, a freewheeling inductor L2, and switch tubes S2, S3, and S5; The DC-AC inverter step-down unit includes an intermediate DC capacitor C1, a switch tube S1, a switch tube S2, a switch tube S4, a switch tube S5, a filter inductor L0 and a voltage stabilizing capacitor C0; In the DC-DC boost conversion unit, the photovoltaic DC input power source V in The positive electrode of the boost inductor L1 is respectively connected to the positive end of the energy storage capacitor C2 and one end of the boost inductor L1, the other end of the boost inductor L1 is respectively connected to the anodes of the diode D1 and the diode D2, the negative end of the energy storage capacitor C2 is respectively connected to the cathode of the diode D1 and one end of the freewheeling inductor L2, the other end of the freewheeling inductor L2 is respectively connected to the cathode of the diode D2, the drain of the switch tube S3 and the source of the switch tube S5, the drain of the S5 switch tube is connected to the positive end of the DC capacitor C1, and the source of the switch tube S2 is connected to the negative end of the DC capacitor C1; In the DC-AC inverter step-down unit, the positive end of the intermediate capacitor C1 is connected to the drain of the switch tube S4 and the drain of the switch tube S5 respectively, the secondary end of the intermediate capacitor C1 is connected to the drain of the switch tube S1 and the drain of the switch tube S2, the source of the switch tube S4 and the drain of the switch tube S1 are both connected to the positive end of the filter inductor L0, the negative end of the filter inductor L0 is connected to the positive end of the voltage stabilizing capacitor C0, and the negative end of the voltage stabilizing capacitor C0 is connected to the drain of the switch tube S2 and the source of the switch tube S3.

2. The common ground type non-isolated photovoltaic inverter according to claim 1, characterized in that: The common ground type non-isolated photovoltaic inverter includes 6 working modes: Working mode 1: switch tubes S2, S3 and S4 are turned on, and switch tubes S1 and S5 are turned off; In working mode 1, the photovoltaic DC output power v in Under the action of , the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, the diode D1 and the diode D2 are turned off by the reverse voltage, and the intermediate capacitor C1 transfers the energy to the AC output side; Working mode 2: switch tubes S1, S2 and S3 are turned on, and switch tubes S4 and S5 are turned off; In working mode 2, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged. Working mode 3: switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off due to reverse voltage. In working mode 3, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit; Working mode 4: switch tubes S1, S3 and S5 are turned on, switch tubes S2 and S4 are turned off, and D1 is turned off due to reverse voltage. In working mode 4, the photovoltaic DC power supply charges the boost inductor L1 and the freewheeling inductor L2, and the current gradually increases; the intermediate capacitor C1 transfers energy to the AC output side, and the current of the filter inductor L0 continues to increase; Working mode 5: the switch tubes S1, S2 and S3 are turned on, and the switch tubes S4 and S5 are turned off; In working mode 5, the current of the boost inductor L1 and the freewheeling inductor L2 continues to rise, and the filter inductor L0 provides energy on the AC output side. At the same time, the current decreases, and the intermediate capacitor C1 is neither charged nor discharged. Working mode 6: switch tubes S1, S2 and S5 are turned on, switch tubes S3 and S4 are turned off, and D1 is turned off due to reverse voltage. In working mode 6, the currents of the boost inductor L1, the freewheeling inductor L2 and the filter inductor L0 continue to decrease, and the photovoltaic DC input power charges the intermediate capacitor C1 through the boost unit.

3. The common ground type non-isolated photovoltaic inverter according to claim 1, characterized in that: The common-ground type non-isolated photovoltaic inverter also includes a control system; The control module includes an independent first submodule and a second submodule, wherein the first submodule and the second submodule respectively control a DC-DC boost duty cycle d1 and a DC-AC inverter buck duty cycle d2; The DC-DC boost duty cycle d1 is used to control the DC-DC boost conversion unit, increase the voltage of the intermediate capacitor C1 to a set target value, and eliminate the even-order current ripple on the DC side; The DC-AC inverter step-down duty cycle d2 is used to control the DC-AC inverter step-down unit to invert and step down the voltage of the intermediate capacitor C1 to AC power with an effective value of 220V and 50Hz, and eliminate odd harmonics of the output AC power.

4. The common ground type non-isolated photovoltaic inverter according to claim 3, characterized in that: The method for obtaining the DC-DC boost duty cycle d1 comprises the following steps: S1, sampled output AC voltage v o , and then pass through the band-stop filter to remove the secondary ripple and the reference value of the output voltage v oref *Compare and get the difference; S2, send the comparison difference to the proportional integrator PI for adjustment to obtain the DC average power control duty cycle d f ; S3, the sampled output voltage v o After the second and fourth bandpass filters respectively, and multiplied by the proportional coefficients K1 and K2 respectively, the corresponding duty ratios d1 of the second and fourth harmonics of the DC-DC boost duty ratio d1 are obtained. h1 ,d h2 ; S4, the duty cycle d f ,d h1 and d h2 Add them together to get the required DC-DC boost duty cycle d1 after filtering out the second and fourth harmonics.

5. The common ground type non-isolated photovoltaic inverter according to claim 3, characterized in that: The method for obtaining the DC-AC inverter step-down duty cycle d2 comprises the following steps: A1, sampled output AC voltage v o As the α axis, and the AC voltage v o The phase shift of one quarter cycle is used as the β axis; A2, after αβ0 / dq transformation, we get v o The d-axis component v od and the q-axis component v oq ; A3, take 1 as v od The reference value of v is 0. oq The reference value of v od Compared with 1, v oq After comparing with 0, the difference is sent to the proportional integral controller PI to obtain the control signal components u on the d-axis and q-axis szd and u szq , and obtain the modulation wave u of fundamental power control through αβ0 / dq inverse transformation szα *; A4, multiplying the sampled AC output voltage vo by the sin and cos functions of odd frequencies respectively as dq decoupling; A5, pass the dq decoupling result through the low-pass filter LPF, make a difference with 0, and send the difference to the proportional integral controller PI respectively; A6. Multiply the output of the integral controller PI by the cosine and sin functions respectively as the dq inverse transformation, and add up the outputs to obtain the voltage V of the odd harmonics. h3 ; A7, u szα * Add odd harmonics of V h3 Get the port voltage reference value u r , will u r Take the absolute value to get the DC-AC inverter step-down duty cycle d2.

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